1. Field of the Invention
The present invention relates to fastener means for joining two metal panel frames and more particularly, to a captive screw, which has an endless isolation groove defined on the top wall of the head of the lock screw within the cap-shaped knob around the top protrusion of the lock screw, facilitating attachment of a hand tool to the top protrusion of the lock screw so that the lock screw can be driven by the user conveniently with the hand tool into a mounting screw hole of a second panel frame to lock the second panel frame to a first panel frame that carries the captive screw.
2. Description of the Related Art
When fastening panel frames together, a fastening screw assembly formed of a knob, a barrel and a screw is usually used. During installation, the screw and the barrel are secured to the first panel frame, and then the knob is rotated to drive the screw into the second panel frame, and then a hand tool is used to fasten tight the screw, affixing the first and second panel frames together. This design of fastening screw assembly can be used in a machine tool to join panel frames together. In a machine tool where panel frames are to be fastened together are the locations of the power drive and speed-adjustment unit. Further, the power drive and speed-adjustment unit of a machine toll are common arranged inside a housing. When the power drive or speed-adjustment unit fails, or when an adjustment of the speed of the power drive or speed-adjustment unit is necessary, the panel frames must be unlocked. When unlocked the panel frames, the component parts of the fastening screw assembly may fall from the panel frames and missed accidentally, affecting further re-installation operation.
To facilitate detachable installation of panel frames and to avoid missing of component parts, captive screws are developed. Conventional captive screws are commonly formed of a cap member, a screw, a spring member and a mounting barrel. During application, the mounting barrel is affixed to a mounting through hole on a first panel frame to hold the screw, the spring member and the cap member in place, allowing the screw to be detachably driven into a mounting screw hole on a second panel frame to detachably lock the second panel frame to the first panel frame. These captive screws prevent falling of component parts from the first panel frame after separation of the first panel frame from the second panel frame.
As shown in FIGS. 7 and 8, the lock screw A1 of the fastening device A has a head A11 of a lock screw A1, a threaded shank A12 located on the bottom side of the head A11, a top protrusion A13 raised from the top wall of the head A11, an embossment A111 located on the periphery of the head A11 and a tool groove A131 located on the top side of the top protrusion A13. The knob A14 of the fastening device A is directly molded from a plastic material on the periphery of the head A11. The knob A14 defines therein an accommodation space A141 around the lock screw A1 for accommodating the mounting barrel B and spring member C of the captive screw. The spring member C is sleeved onto the threaded shank A12 of the lock screw A1 and stopped between the head A11 of the lock screw A1 and a step in an inside space B1 of the mounting barrel B. When assembled, the knob A14 is axially slidably coupled to the mounting barrel B, avoiding falling of the lock screw A1 and the spring member C out of the mounting barrel B. However, this prior art design still has drawbacks as follows:    1. The knob A14 of the fastening device A is directly molded from a plastic material on the periphery of the head A11 of the lock screw A1. During molding of the knob A14 on the head A11 of the lock screw A1, the molten plastic material may flow to the tool groove A131, causing the tool groove A131 to be fully or partially clogged with the plastic material. If the tool groove A131 is clogged, rotating the fastening device A with a hand tool will be difficult or not possible, and the captive screw will become a defective product.    2. The top protrusion A13 of the lock screw A1 can simply be processed to provide a crossed tool groove A131. If the top protrusion A13 of the lock screw A1 is processed to provide a straight tool groove, the molding plastic material may flow to the straight tool groove easily during molding of a knob on the head of the lock screw. Thus, the design of the tool groove is limited, not able to facilitate in-situ installation.
Therefore, it is desirable to provide a captive screw design that facilitates fabrication and, prevents overflow of the applied molten plastic material during molding of the knob on the head of the lock screw.